I've just thumbed through Oliver Twist leeneia, and strangely enough, Dickens doesn't depict Cockney speech by substituting f for th. And one would imagine the urchins in Fagin's den would have spoken Cockney like the very Devil!

However, the accent of East London was probably heard for hundreds of years, while rhyming slang became current in the nineteenth century.

I spoke with rather a Cockney accent (with a West London twist) when I was a girl, much to my father's annoyance! My sister and I dropped our aitches and used f for th, but tried to modify our speech if he was around.